Pin mesa diodes with over-current protection

ABSTRACT

A system includes a pixel including a diffusion layer in contact with an absorption layer. The diffusion layer and absorption layer are in contact with one another along an interface that is inside of a mesa. A trench is defined in the absorption layer surrounding the mesa. An overflow contact is seated in the trench.

BACKGROUND 1. Field

The present disclosure relates to diodes, and more particularly tophotodiodes such as used in pixels for imaging.

2. Description of Related Art

The lower the dark current of photodiodes in an imaging device, thebetter is the image quality. Similarly, the higher the sensitivity ofphotodiodes in imaging devices, the better is the image quality. At highlight levels, the excess photons generate high current, which may damagea detector and/or put more stress on the read-out integrated circuit(ROIC). A photodetector array (PDA) with mesa structures improvemodulation transfer function (MTF) of an imaging device.

The conventional techniques have been considered satisfactory for theirintended purpose. However, there is an ever present need for improvedsystems and methods for photodiodes used in imaging devices. Thisdisclosure provides a solution for this need.

SUMMARY

A system includes a pixel including a diffusion layer in contact with anabsorption layer. The diffusion layer and absorption layer are incontact with one another along an interface that is inside of a mesa. Atrench is defined in the cap layer and absorption layer surrounding themesa. An overflow contact is seated in the trench.

The overflow contact can surround the mesa. The pixel can be one of aplurality of similar pixels arranged in a grid pattern, wherein thetrench separates each pixel from adjacent ones of the pixels. Theoverflow contact can be seated in a base of the trench, forming anoverflow contact grid wherein the pixels are in the spaces of theoverflow contact grid between intersecting lines of the overflow contactgrid.

The trench can have a base between sidewalls of the trench, wherein theoverflow contact is seated on the base, and wherein there is lateralclearance on the base between the overflow contact and each of thesidewalls. The overflow contact can seat against a transparentconductive oxide (TCO) that in turn contacts the absorption layer. TheTCO can include multiple layers of ZnO, TiO₂ and/or Indium Tin Oxide(ITO). The TCO can be formed in a gap within a SiNx layer. A contactmetal can be electrically connected to the diffusion layer, configuredto electrically connect the diffusion layer to a read-out integratedcircuit (ROIC).

The overflow contact can be metallic. A cap layer can be deposited onthe absorption layer opposite a substrate. The cap layer can includeInP. A SiNx layer can be included over the cap layer. The SiNx layer candirectly contact the absorption layer within the trench. Ananti-reflective layer can be deposited on the substrate opposite theabsorption layer. The absorption layer can include InGaAs, wherein thepixel is sensitive to illumination in infrared wavelengths. It is alsocontemplated that the absorption layer can include Si, wherein the pixelis sensitive to illumination in visible light wavelengths.

A method includes forming a trench in an absorption layer of a pixelarray, wherein the trench is formed in a grid pattern surroundingrespective pixels of the pixel array, wherein each pixel is surroundedthe intersecting lines of the grid. The method includes forming anoverflow contact in the trench, following the grid pattern.

The method can include depositing a SiNx layer on the absorption layerafter forming the trench, wherein forming the overflow contact in thetrench is performed after depositing the SiNx layer. Forming theoverflow contact in the trench can include forming the overflow contacton a transparent oxide layer (TCO) formed in a gap in the SiNx layer.

These and other features of the systems and methods of the subjectdisclosure will become more readily apparent to those skilled in the artfrom the following detailed description of the preferred embodimentstaken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

So that those skilled in the art to which the subject disclosureappertains will readily understand how to make and use the devices andmethods of the subject disclosure without undue experimentation,preferred embodiments thereof will be described in detail herein belowwith reference to certain figures, wherein:

FIG. 1 is a schematic cross-sectional elevation view of an embodiment ofa system constructed in accordance with the present disclosure, showingthe mesa and trench; and

FIG. 2 is a schematic plan view of the system of FIG. 1, showing thegrid pattern for a plurality of pixels.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made to the drawings wherein like referencenumerals identify similar structural features or aspects of the subjectdisclosure. For purposes of explanation and illustration, and notlimitation, a partial view of an embodiment of a system in accordancewith the disclosure is shown in FIG. 1 and is designated generally byreference character 100. Other embodiments of systems in accordance withthe disclosure, or aspects thereof, are provided in FIG. 2, as will bedescribed. The systems and methods described herein can be used toreduce excess current, increase sensitivity, and reduce stress onread-out integrated circuits (ROICs) in imaging devices.

The system 100 includes a pixel 102 including a diffusion layer 104 incontact with an absorption layer 106. The diffusion layer 104 andabsorption layer 106 are in contact with one another along an interface108 that is inside of a mesa 109. A trench 110 is defined in theabsorption layer 106 surrounding the mesa 109. An overflow contact 112is seated in the trench 110.

With reference now to FIG. 2, the pixel 102 is one of a plurality ofsimilar pixels 102 arranged in a square tiled grid pattern 114, aportion of which is indicated with reference character 114 and theassociated phantom lines in FIG. 2. The trench 110 separates each pixel102 from adjacent ones of the pixels 102. The overflow contact 112surrounds each mesa 109. The overflow contact 112 is seated in a base116 of the trench 110 (as identified in FIG. 1), forming an overflowcontact grid 118. A portion of the overflow contact grid 118 isindicated in FIG. 2 with reference character 118 and the associatedphantom lines. The pixels 102 are in the spaces of the overflow contactgrid 118 between intersecting lines 120 of the overflow contact grid118.

Referring again to FIG. 1, the base 116 of the trench 110 is locatedbetween sidewalls 122 of the trench 110. The overflow contact 112 isseated on the base 116 and there is lateral clearance 124 on the base116 between the overflow contact 112 and each of the sidewalls 122. Theoverflow contact seats against a transparent conductive oxide (TCO) 126that in turn contacts the absorption layer 106. The TCO 126 can includemultiple layers of ZnO, TiO₂ and/or Indium Tin Oxide (ITO). The filmresistivity of the TCO 126 can be designed based on detector workingconditions, which can be implemented by adjusting the doping level ofthe TCO 126. The TCO 126 is formed in a gap 128 within a SiNx layer 130.For each pixel 102 (e.g. as shown in FIG. 2), a contact metal 132electrically connects between the diffusion layer 104 and a read-outintegrated circuit (ROIC) 134. The TCO 126 normally acts as aninsulator, but when current reaches a predetermined maximum level, theresistance barrier of the TCO 126 breaks and excess current can flowthrough the TCO to a common current sink 144 on the ROIC 134.

The overflow contact 112 can be metallic. A cap layer 136, e.g. of InP,can be deposited on the absorption layer opposite a substrate 138, whichcan be of InP or any other suitable material. The SiNx layer 130 isincluded, located over the cap layer. The SiNx layer 130 directlycontacts the absorption layer 106 within the trench 110. Ananti-reflective layer 140 can optionally be deposited on the substrate138 opposite the absorption layer 106. The absorption layer 106 caninclude InGaAs, e.g. making the pixel 102 sensitive to illumination ininfrared wavelengths. It is also contemplated that the absorption layer106 can include Si, e.g. making the pixel 102 is sensitive toillumination in visible light wavelengths. Those skilled in the art willreadily appreciate that any other suitable material can be used toprovide sensitivity in any other suitable wavelengths.

A method includes forming a trench, e.g. trench 110, in an absorptionlayer, e.g. absorption layer 106, of a pixel array (e.g. the array ofpixels 102 of the square tiled grid pattern 114 in FIG. 2). The trenchis formed in a grid pattern, e.g., grid pattern 114 of FIG. 2)surrounding respective pixels of the pixel array, wherein each pixel issurrounded by the intersecting lines of the grid (e.g. grid lines 142 ofFIG. 2). The method includes forming an overflow contact, e.g., overflowcontact 112, in the trench, following the grid pattern. The method caninclude depositing a SiNx layer, e.g. SiNx layer 130) on the absorptionlayer after forming the trench, wherein forming the overflow contact inthe trench is performed after depositing the SiNx layer. Forming theoverflow contact in the trench can include forming the overflow contacton a transparent oxide layer (TCO), e.g., TCO 126, formed in a gap, e.g.gap 128, in the SiNx layer. The diffusion layer 104 is the P portion ofa PIN diode, the absorption layer 106 is the I portion of the PIN diode,and the substrate 138 is the N portion of the PIN diode.

The methods and systems of the present disclosure, as described aboveand shown in the drawings, provide for reduced excess current, increasedsensitivity, and reduced stress on read-out integrated circuits (ROICs)in imaging devices. This can improve image quality and can reduce ROICdesign requirements and signal processing complications relative totraditional configurations. While the apparatus and methods of thesubject disclosure have been shown and described with reference topreferred embodiments, those skilled in the art will readily appreciatethat changes and/or modifications may be made thereto without departingfrom the scope of the subject disclosure.

What is claimed is:
 1. A system comprising: a pixel including a diffusion layer in contact with an absorption layer, wherein the diffusion layer and absorption layer are in contact with one another along an interface that is inside of a mesa; a trench defined in the absorption layer surrounding the mesa; and an overflow contact seated in the trench.
 2. The system as recited in claim 1, wherein the overflow contact surrounds the mesa.
 3. A system as recited in claim 1, wherein the pixel is one of a plurality of similar pixels arranged in a grid pattern, wherein the trench separates each pixel from adjacent ones of the pixels, and wherein the overflow contact is seated in a base of the trench, forming an overflow contact grid wherein the pixels are in the spaces of the overflow contact grid between intersecting lines of the overflow contact grid.
 4. The system as recited in claim 1, wherein the overflow contact is metallic.
 5. The system as recited in claim 1, further comprising a cap layer deposited on the absorption layer opposite a substrate.
 6. The system as recited in claim 5, wherein the cap layer includes InP.
 7. The system as recited in claim 5, further comprising a SiNx layer over the cap layer.
 8. The system as recited in claim 7, wherein the SiNx layer directly contacts the absorption layer within the trench.
 9. The system as recited in claim 5, further comprising an anti-reflective layer deposited on the substrate opposite the absorption layer.
 10. The system as recited in claim 1, wherein the trench has a base between sidewalls of the trench, wherein the overflow contact is seated on the base, and wherein there is lateral clearance on the base between the overflow contact and each of the sidewalls.
 11. The system as recited in claim 1, further comprising a contact metal electrically connected to the diffusion layer, configured to electrically connect the diffusion layer to a read-out integrated circuit (ROIC).
 12. The system as recited in claim 11, further comprising the ROIC connected in electrical communication with the contact metal.
 13. The system as recited in claim 1, wherein the overflow contact seats against a transparent conductive oxide (TCO) that in turn contacts the absorption layer.
 14. The system as recited in claim 13, wherein the TCO includes multiple layers of ZnO, TiO2 and/or Indium Tin Oxide (ITO).
 15. The system as recited in claim 11, wherein the (TCO) is formed in a gap within a SiNx layer.
 16. The system as recited in claim 1, wherein the absorption layer includes InGaAs, and wherein the pixel is sensitive to illumination in infrared wavelengths.
 17. The system as recited in claim 1, wherein the absorption layer includes Si, and wherein the pixel is sensitive to illumination in visible light wavelengths.
 18. A method comprising: forming a trench in an absorption layer of a pixel array, wherein the trench is formed in a grid pattern surrounding respective pixels of the pixel array, wherein each pixel is surrounded the intersecting lines of the grid; and forming an overflow contact in the trench, following the grid pattern.
 19. The method as recited in claim 18, further comprising depositing a SiNx layer on the absorption layer after forming the trench, wherein forming the overflow contact in the trench is performed after depositing the SiNx layer.
 20. The method as recited in claim 19, wherein forming the overflow contact in the trench includes forming the overflow contact on a transparent oxide layer (TCO) formed in a gap in the SiNx layer. 